This invention relates to a method for producing a welded joint.
A welded joint produced by a method of the prior art is shown schematically in cross section in FIG. 1. The process for producing the welded joint of the structure shown in FIG. 1 can be explained by referring to FIG. 2 in which a deposited metal 1 provided by the preceding welding operation and a deposited metal 2 provided by the next following welding operation are positioned at the right and left sides in lapping relation.
By vertically applying layers of the deposited metal which are first arranged at the right and left sides in lapping relation, a groove of a V-shape of a weld preparation formed between components 3 and 4 to be welded is deposited with a filler metal.
A welded joint produced by the process described above includes layers of deposited metal which each have a large thickness in the vertical direction. Because of the great thickness, it is impossible to temper an underlying layer of deposited metal in its entirety by the heat generated when a layer of deposited metal located immediately over the underlying layer of deposited metal is formed. Therefore, it is impossible to cause transformation of a coarse metal structure, such as a columnar metal structure, which is formed when the underlying layer of deposited metal is provided, into a fine metal structure in its entirety.
More specifically, in welded joints of the prior art, it is only those parts of the layers of deposited metal which overlap one another that are tempered and transformed into a fine metal structure. Moreover, inasmuch as the layers of deposited metal each have a large thickness, gases existing in the molten metal are unable to escape by diffusion to outside at the time the deposited metal is still in molten state, resulting in entrapment of these gases in the deposited metal.
Such being the case, conventional welded joints have a metal structure such that it has holes therein and includes a coarse metal structure which causes a reduction in the toughness of the welded joints at low temperatures. Thus welded joints of the prior art have had the disadvantages of being unsatisfactory in performance.